The invention generally relates to methods for treating a semiconductor material. More particularly, the invention relates to methods for treating a semiconductor layer used in thin-film photovoltaic devices.
Thin film photovoltaic devices typically include a plurality of semiconductor layers disposed on a transparent substrate, wherein a first semiconductor layer serves as a window layer and a second semiconductor layer serves as an absorber layer. The window layer allows the penetration of solar radiation to the absorber layer, where the optical energy is converted to usable electrical energy. In certain configurations, thin film photovoltaic devices may further include an additional semiconductor layer interposed between the window layer and the absorber layer that may function as an intrinsic layer. Cadmium telluride/cadmium sulfide (CdTe/CdS) heterojunction-based photovoltaic devices are one such example of thin film solar cells, where a cadmium telluride (CdTe)-based semiconductor layer may function as an intrinsic layer or an absorber layer.
However, CdTe-based photovoltaic devices typically demonstrate relatively low power conversion efficiencies, which may be attributed to a relatively low open circuit voltage (Voc) in relation to the band gap of the material which is due, in part, to the low effective carrier concentration and short minority carrier lifetime in CdTe. Effective carrier concentration of CdTe may be improved by doping with p-type dopants.
Further issues with improving the device efficiency of CdTe solar cells include the high work function of CdTe and high back-contact resistance at the interface between CdTe and metal-based back contact layer. The back-contact resistance may be improved by increasing the carrier concentration at the back interface. For example, for a p-type CdTe material, increasing the carrier concentration amounts to increasing the p-type carriers in the CdTe material to form an “ohmic contact layer” on the backside of the CdTe layer, which is in contact with the back contact layer.
Typical methods employed to form the ohmic layers or for doping the absorber layer include etching of the CdTe layers and incorporation of copper into back-end of line processing of the absorber layer. However, it may be difficult to control the amount of copper incorporated in the bulk and in the back interface, using a typical CdTe processing method. Further, photovoltaic devices manufactured using the typical methods may include a high copper content at the back-interface, which may adversely affect the long-term stability. Furthermore, etching of the CdTe layer using conventional etching agents may lead to removal of CdTe material from the surface, and selective etching of grain boundaries, resulting in increased defects.
Thus, there is a need for improved methods of processing semiconductor layers. Further, there is a need for improved photovoltaic device configurations including the semiconductor layers.